In a conventional metal oxide semiconductor field effect transistor (MOSFET), the source, channel, and drain structures are constructed adjacent to each other within the same plane. Typically, a gate dielectric is formed on the channel area and a gate electrode is deposited on the gate dielectric. The transistor is controlled by applying a voltage to the gate electrode, thereby allowing a current to flow through the channel between source and drain.
An alternative to methods of building planar MOSFETs has been proposed to help alleviate some of the physical barriers to scaling down existing designs. The alternative method involves a construction of three dimensional MOSFETs, in the form of a multi-gate transistor such as a dual-gate field effect transistor (FinFET) or a tri-gate transistor, as a replacement for the conventional planar MOSFET.
Three-dimensional transistor designs such as the dual-gate FinFET and the tri-gate transistor allow tighter packing of the same number of transistors on a semiconductor chip by using vertical or angled surfaces for the gates. A tri-gate transistor comprises three equal length gates situated on three exposed surfaces of a body whereas a dual-gate transistor comprises two equal length gates situated along the sides of a narrow body.
An overall contact resistance of the tri-gate transistor is a function of a contact resistance contributed by the top gate and a contact resistance contributed by each of the two side gates. The contact resistance at each gate is determined in part by the contact area of the source and drain, materials used at the interface of the source and drain regions, such as a silicide layer, and the manner in which those materials interface. The silicide layer may be formed on the source and drain regions for the top and side gates of a multi-gate transistor to reduce the contact resistance, thereby increasing a transistor current. The contact resistance can increase when a portion of the silicide material is blocked or is otherwise prevented from contacting a source or drain region.